In vitro lymphocyte activation by mitogens and antigen was analyzed by laser transmission cytometry. The new method directly measures cellular events. The formation of lymphoblasts and increase in total cells are quantitatively related to conventional methods of pulse labelling. Statistically this approach appears to be more reproducible. Immunotherapeutic effect against B16 melanoma was evaluated. Heterologous antibodies against B16 cells produced in rabbits had an indirect immunofluorescence titer of 1280. Passive immunization of C57BL/6 mice reduced tumor incidence from 100% in the controls of 21.2% (7/33). The mechanism of the therapeutic effect is not related to complement mediated cytotoxicity, since the antibodies failed to fix mouse complement in vitro.